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arXiv:2608.23768

Multi-hour stable trapping and threshold steady state of non-laser-coolable ions with ultracold atoms in a hybrid trap

Satyabrata Baidya, Sourav Dutta

Published Aug 24, 2026• 0 citations
arXivHTMLPDF

Abstract

We report multi-hour stable trapping and steady state of non-laser-coolable Cs$^{+}$ ions in a linear Paul trap, achieved via resonant charge-exchange (RCE) cooling with a precisely centered ultracold Cs cloud. Without cold atoms, all ions are lost within 3 minutes. With centered cold atoms, RCE cooling establishes a stable population of ions for 6 hours with no measurable decay -- more than two orders of magnitude longer hold times compared to prior hybrid atom-ion systems. We observe a threshold behavior in the steady-state number of ions ($N_{s}$): initial ion loadings above $N_{s}$ ions converge downward to $N_{s}$ ions and remain constant thereafter; initial loadings below $N_{s}$ ions show no measurable decay. The dynamics is governed by a competition between ion-ion rf heating and ion-atom collisional cooling. The prolonged, simultaneous ion-atom trapping suggests an upper bound on the three-body recombination-induced Cs$_{2}^{+}$ formation rate constant: $k_{3} << 1.4 \times 10^{-25} cm^{6} s^{-1}$. This remarkable realization of long-lived steady state overcomes a critical limitation of prior hybrid atom-ion systems and enables extended studies of ultracold ion-neutral chemistry, rare inelastic collisions, and sympathetic cooling routes for complex molecular ions.

DOI
10.48550/arxiv.2608.23768

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We report multi-hour stable trapping and steady state of non-laser-coolable Cs+^{+}+ ions in a linear Paul trap, achieved via resonant charge-exchange (RCE) cooling with a precisely centered ultracold Cs cloud. Without cold atoms, all ions are lost within 3 minutes. With centered cold atoms, RCE cooling establishes a stable population of ions for 6 hours with no measurable decay -- more than two orders of magnitude longer hold times compared to prior hybrid atom-ion systems. We observe a threshold behavior in the steady-state number of ions (NsN_{s}Ns​): initial ion loadings above NsN_{s}Ns​ ions converge downward to NsN_{s}Ns​ ions and remain constant thereafter; initial loadings below NsN_{s}Ns​ ions show no measurable decay. The dynamics is governed by a competition between ion-ion rf heating and ion-atom collisional cooling. The prolonged, simultaneous ion-atom trapping suggests an upper bound on the three-body recombination-induced Cs2+_{2}^{+}2+​ formation rate constant: k3<<1.4×10−25cm6s−1k_{3} << 1.4 \times 10^{-25} cm^{6} s^{-1}k3​<<1.4×10−25cm6s−1. This remarkable realization of long-lived steady state overcomes a critical limitation of prior hybrid atom-ion systems and enables extended studies of ultracold ion-neutral chemistry, rare inelastic collisions, and sympathetic cooling routes for complex molecular ions.
The two most widely used ion cooling methods are laser cooling and sympathetic cooling by elastic collisions (ECs). Here, we demonstrate another method of cooling ions that is based on resonant charge exchange (RCE) between the trapped ion and the ultracold parent atom. Specifically, trapped Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ ions are cooled by collisions with cotrapped, ultracold Cs atoms and, separately, by collisions with cotrapped, ultracold Rb atoms. We observe that the cooling of Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ ions by Cs atoms is more efficient than the cooling of Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ ions by Rb atoms. This signals the presence of a cooling mechanism apart from the elastic ion-atom collision channel for the Cs-Cs+\mathrm{C}{\mathrm{s}}^{+}Cs+ case, which is cooling by RCE. The efficiency of cooling by RCE is experimentally determined and the per-collision cooling is found to be two orders of magnitude higher than cooling by EC. The result provides the experimental basis for future studies on charge transport by electron hopping in atom-ion hybrid systems.
Abstract We present a series of recent experimental and numerical results which allow us to propose and explain the mechanisms for collisional cooling of a trapped ion by sequential binary collisions with atoms. Our experiments in hybrid atom-ion traps study the cooling of ions when the reservoir of the coolant atoms is spatially localized at the centre of the ion trap. Under these conditions the widely and long held expectation that for the ion to collisionally cool, the atom must be of lighter mass (i.e. mI > mA ) is overturned. Instead we show ion cooling for mI ⩽ mA in addition to mI > mA , and explain why earlier work establishing mass ratios does not apply to the experiments in hybrid traps. Further, for cooling of an ion A+ by the parent atom A, the mechanism of resonant charge exchange (RCE) allows for extremely efficient cooling of the ion A+. This mechanism is demonstrated by comparing the cooling rate of an ion with a localized ensemble of its parent atoms and with a lighter atomic species, where in the latter case the RCE mechanism does not exist. The difference in measured cooling rates for a given number density of the atoms and the theoretically calculated difference between elastic and RCE cross sections are used to show that the cooling efficiency per collision with RCE is much greater than the cooling by elastic collision. We conclude with some perspectives and prospects of future experiments.
A dual hybrid ion–neutral trap has been built to study collisions between a cloud of ultracold atoms and small numbers of co-trapped atomic or molecular ions, in a common volume. Ultracold Na atoms are trapped in a vapour-cell magneto-optical trap (MOT), via a single-mode frequency-stabilized ring-dye laser, with repumping. A linear rf quadrupole Paul ion trap, centred on the MOT, co-traps selected atomic or molecular ions, for example Ca+. Control of the initial ion temperature involves laser cooling Ca+ ions via the resonant Fraunhofer K line at 397 nm. We showed the rf ion trap can operate simultaneously with the MOT without destroying the ultracold atom cloud. The goal is to investigate the physics of ion–neutral sympathetic cooling, the cooling by neutrals of translationally cold and vibrorotationally hot molecular ions (e.g. Na2 +(v*, J*)), and the observation of other ion–neutral processes near 0 K, of potential interest in astrophysics.
Linear Paul traps use radio frequency fields to confine ions for long times in deep potentials. The simplicity and versatility of these devices has led to their use in a wide variety of fields, from mass spectroscopy to quantum information. Within the past fifteen years, beginning with pioneering work at UConn, Paul traps have been combined with atom traps to form hybrid atom-ion traps. Hybrid traps serve as platforms for measuring ion-atom interaction rates, sympathetically cooling atomic and molecular ions, and studying the behavior of trapped ions. This work discusses simulations, analytical theory, and experiments in a hybrid trap that show a previously unknown dynamical effect in a population of trapped ions. The steady state trapped ion number in a Paul trap is non-monotonic with the loading rate of the Paul trap. This effect is universal to both Linear and 3D Paul traps and is likely to be found in other types of ion traps as well because it is the result of the Coulomb interaction in the presence of a trap. This behavior also affects the decay of ions from the trap. Understanding the loading and decay behavior from a saturated Linear Paul trap is important for the study of ion-atom interactions in a hybrid trap when the ions lack optical transitions.
Abstract Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.
Long-range polarization forces between ions and neutral atoms result in large elastic scattering cross sections (e.g., $\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}\phantom{\rule{-0.16em}{0ex}}{10}^{6}\phantom{\rule{0.28em}{0ex}}\mathrm{a}.\mathrm{u}.$ for Na\mathrm{Na}Na-Na+{\mathrm{Na}}^{+}Na+ or Na\mathrm{Na}Na-Ca+{\mathrm{Ca}}^{+}Ca+ at cold and ultracold temperatures). This suggests that a hybrid ion-neutral trap should offer a general means for significant sympathetic cooling of atomic or molecular ions. We present simion 7.0 simulation results concerning the advantages and limitations of sympathetic cooling within a hybrid trap apparatus consisting of a linear rf Paul trap concentric with a Na\mathrm{Na}Na magneto-optical trap (MOT). This paper explores the impact of various heating mechanisms on the hybrid system and how parameters related to the MOT, Paul trap, number of ions, and ion species affect the efficiency of the sympathetic cooling.